The Structure of Adsorbed Species on Immobilized Amines in CO2

Aug 18, 2016 - Adsorbed H2O mainly on secondary amine enhanced CO2 capture capacity by increasing accessibility of amine sites and promoting the forma...
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The Structure of Adsorbed Species on Immobilized Amines in CO2 Capture: An in Situ IR Study Jie Yu and Steven S. C. Chuang* Department of Polymer Science, The University of Akron, Akron, Ohio 44325-3909, United States S Supporting Information *

ABSTRACT: The nature and structure of adsorbed CO2 on immobilized amine sorbent in the presence and absence of H2O vapor have been studied by in situ infrared spectroscopy. CO2 adsorbed on the primary amine as ammonium carbamate and on the secondary amine as carbamic acid. Adsorbed H2O mainly on secondary amine enhanced CO2 capture capacity by increasing accessibility of amine sites and promoting the formation of carbamic acid. The binding strength of the adsorbed species increased in the order: carbamic acid < adsorbed H2O < paired carbamic acid; ammonium carbamate < ammonium chloride. Flowing argon over the amine sorbent at 50 °C removed weakly adsorbed H2O and carbamic acid from the secondary amine sites. Raising temperature is required to completely regenerate sorbent by removing strongly adsorbed ammonium carbamate from the primary amine sites and paired carbamic acid. The results of this study clarify the role of H2O vapor in amine-sorbents for CO2 capture and provide a molecular basis for the design of the sorbents and operation of amine-based CO2 capture processes.

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INTRODUCTION Coal-fired power generation will remain as a major CO2 emission source for the next several decades because of its reliability as well as the lack of significant progress in development of cost-effective renewable and energy storage technologies. Coal-fired power plants contributed 71% of the CO2 emission from the U.S. electric power sectors in 2015.1 A U.S. Department of Energy (DOE) study suggested that incorporation of current monoethanol amine (MEA) technology could lead to an increase of the cost of electricity (COE) from $82 to $133/MWh.2 The high cost of the MEA process can be attributed to the following: (i) the energy needed for MEA regeneration, (ii) the corrosiveness of aqueous amine to equipment, and (iii) the degradation and evaporation loss of MEA. These drawbacks could be alleviated by using immobilized amines which incorporate amines on the surface of porous supports.3−6 Schemes 1 and 2 compare the proposed pathways of CO2amine reactions in aqueous amines and immobilized amines.

Scheme 2. Reaction Mechanism of CO2 with Immobilized Amines

of carbamate decreased as bicarbonate increasing in aqueous MEA and diethanolamine (DEA), confirming that bicarbonate is produced from ammonium carbamate ions.8 Scheme 3, derived from the results of this study, reveals that carbamic acid played an important role in the CO2 capture process. Adsorption of CO2, in the form of bicarbonate and carbamic acid, is highly desirable because of a 1:1 stoichiometric ratio of CO2−amine. H2O vapor, constituting 4−10% of the flue gas stream, has been reported to have either positive or negative impact on the CO2 capture capacity and amine efficiency of immobilized amine sorbents.9−12 Water aided in forming bicarbonate in an aqueous process.13−15 However, bicarbonate had not been unambiguously identified as a major specie on immobilized amine sorbents until a recent study,16 which suggested that bicarbonate could be gradually formed after rapid formation of ammonium carbamate on a sorbent with low amine coverage (1.65 mmol N/g) in the presence of water vapor. The deterioration

Scheme 1. Reaction Mechanism of CO2 with Aqueous Amines

The ammonium−carbamate zwitterion, which has been identified by NMR studies,7 is a precursor to form ammonium carbamate ions. Infrared studies showed that the concentration © XXXX American Chemical Society

Received: June 11, 2016 Revised: August 16, 2016

A

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Energy & Fuels Scheme 3. Proposed Structures of (a) Adsorbed H2O and (b) Adsorbed CO2/H2O on Immobilized Amine Sites

sorbent was characterized by SEM (Hitachi TM3000) and FT-IR (Thermo-Nicolet 6700) with the resolution of 4.0 cm−1 and two coadded scans for each spectrum. CO2 Capture Procedure. The experimental setup for in situ infrared spectroscopic studies of CO2 and CO2/H2O adsorption/desorption, illustrated in Figure 1, includes (a) a gas

effect has been attributed to the competitive adsorption of CO2 and H2O at the same sites on amine sorbent17 and the blockage of amine sites by accumulation of excess H2O.18 The present study is aimed at clarifying the ambiguity of reported effects of H2O on the CO2 capture of the immobilized amine sorbent. In situ infrared spectroscopy was employed for determining the structure, binding sites, and binding energy for adsorbed CO2/H2O, as well as their adsorption/desorption kinetics. CO2 binding energy, and the availability of amine sites, governs the amine-CO2 capture efficiency and sorbent regeneration temperature, while the kinetics of CO2 adsorption/desorption will dictate the time needed for each CO2 capture/regeneration cycle. Tetraethylenepentamine (TEPA) impregnated SiO2 sorbent was chosen to investigate the binding structures of CO2-amine and the kinetics of its association/dissociation. TEPA possesses both primary (i.e., 1°) and secondary (i.e., 2°) amines and exhibits higher thermal stability than other organic amines. Furthermore, TEPA has been widely utilized for impregnation onto numerous other supports, including SiO2, Al2O3,19,20 and carbon.21 Hydrochloric acid (HCl) with an approximate concentration of 1 ppm in flue gas after flue gas desulfurization (FGD) treatment,22 could neutralize the basicity of the amine sites causing the deactivation of sorbent. HCl vapor was added to immobilized amine sorbent for identification of the infrared bands associated with primary/secondary ammonium ions and also qualitatively assess its binding strength to the amines. A fundamental understanding of the structure of adsorbed CO2/H2O, and the interactions of HCl-amine and H2O− CO2-amine as well as their kinetics, will assist in developing effective sorbents and designing CO2 capture processes.

Figure 1. Experimental setup for conducting the CO2 and CO2/H2O adsorption/desorption studies.

manifold with mass flow controllers, (b) a diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) cell, (c) a Pfeiffer QMS 200 quadruple mass spectrometer (MS), and a computer with Labview DAQ module for measurement and control. The DRIFTS cell was filled with 50 mg of TS sorbent. The temperatures of the sorbent bed at bottom and top in the DRIFTS cell were monitored by two thermocouples: TC1 and TC2. CO2 and CO2/H2O capture studies were carried out with the following sequences: (i) pretreatment (i.e., pret.) to remove the preadsorbed CO2 and H2O at 120 °C under 150 cm3/min Ar flow, (ii) adsorption of 15% CO2 or 15% CO2/H2O (i.e., ads.) on amine sorbent at 50 °C by flowing 150 cm3/min CO2 or CO2/H2O for 10 min, (iii) purging (i.e., purge) the sorbent bed with 150 cm3/min Ar flow to remove the gaseous CO2 and weakly adsorbed species for 10 min, and (iv) temperature-programmed desorption (TPD) with the heating rate at 10 °C/min from 50 to 120 °C in flowing 150 cm3/min Ar and holding at

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EXPERIMENTAL SECTION Preparation of TS Sorbent. TEPA/SiO2 (TS) sorbent was prepared through wet impregnation method. A 2.5 g of 20.0 wt % tetraethylenepentamine (TEPA) (Sigma-Aldrich, technical grade) ethanol solution was uniformly dispersed onto 1.0 g of amorphous silica (SiO2, Rhodia Chemicals) support. The resultant mixture was dried at 70 °C for 2 h. The obtained B

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Energy & Fuels Table 1. Physical Characteristics and CO2 Capture Capacity of TS Sorbent CO2 capture capacity (mmol of CO2/g of Sorb.)

amine efficiency (mol of CO2/mol of N)

flue gas

particle size (μm)

TEPA content (wt %)

no. of TEPA layers

Tads

Sads

Wads

Tads

Sads

Wads

15 vol % CO2/air 15 vol % CO2/air + 4 wt% H2O

15−60 15−60

29.0 29.0

3.2 3.2

2.99 3.28

0.89 1.52

2.10 1.78

0.39 0.43

0.12 0.20

0.27 0.23

the external TEPA layers adhered fine SiO2 particles to form larger sorbent aggregates with a size range from 15 to 60 μm, which was observed in the SEM images. CO2 Capture Study of TS Sorbent. Figure 3 provides an overview of the IR and near-IR spectra taken during a CO2/H2O

120 °C for 5 min. We classified those species which can be easily removed by flowing Ar at 50 °C as weakly adsorbed species (Wads) and those removed by TPD as strongly adsorbed species (Sads). H2O vapor was introduced into the inlet line by passing the flow of 15% CO2 via a DI-water saturator kept at room temperature.

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RESULTS AND DISCUSSION Characterization of TS Sorbent. The CO2 capture capacity and amine efficiency as well as the results of sorbent characterizations were listed in Table 1. The CO2 capture capacity and amine efficiency of TS sorbent for total adsorption (Tads) were obtained by weight change method, which was described in Supporting Information (SI), while the strong and weak adsorption (Sads and Wads) were calculated by correlating with the integrated IR peak area under 1860 cm−1 to 1250 cm−1 region, as provided in Figure S1 and Table S1 in the SI. The H2O band at 1650 cm−1 was deconvoluted from those peaks (1860 cm−1 to 1250 cm−1) as integrating the peak area, as shown in Figure S2. The TS sorbent exhibited a relatively high amine efficiency when compared to those reported in literature, summarized in Table S2. The addition of 4% H2O vapor enhanced amine efficiency for strongly adsorbed CO2. SiO2 support with a BET surface area of 160 m2/g exhibited an amorphous particle morphology, shown in the SEM image in Figure 2.

Figure 3. (a) Temperature profiles and normalized IR intensity (norm. IR int.) profiles of gaseous CO2 (2360 cm−1), carbamate (1326 cm−1, NCOO−), and adsorbed H2O (3470 cm−1) during a typical adsorption cycle, (b) IR absorbance spectra of TS sorbent were collected during a CO2/H2O adsorption cycle. Absorbance = log(1/ISB), where ISB is the single-beam spectrum. (c) Difference IR spectrum of adsorbed CO2/H2O was obtained by subtracting APret. from ACO2/H2O,10m.

capture process. The spectrum of the pretreated sorbent (i) in Figure 3b shows the typical spectrum of a polyamine molecule immobilized on a silica support. The specific band assignments with supporting references are summarized in Table 2. Switching the flow from Ar to 15% CO2/H2O, as spectrum (ii) shows, resulted in a sharp increase in the IR intensity of gaseous CO2 at 2360 cm−1 (R branch) and 2340 cm−1 (P branch), produced changes in contours of C−H stretching at 2815 cm−1 and 2925 cm−1, N−H stretching at 3288 and

Figure 2. SEM and EDX images for SiO2 support and TS sorbent.

According to calculations based upon DFT modeling of a TEPA molecule, the amine impregnated sorbent gave approximate 3.2 layers of TEPA molecules on the SiO2 support, assuming an uniform distribution of the TEPA molecules on the inner porous SiO2 surface.4,23 The distribution of the N element in TEPA in the EDX mapping image revealed that C

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Energy & Fuels 3367 cm−1, their combination at 6489, 6554, and 4925 cm−1 as well as their bending bands in the 1100−1800 cm−1 region.

Purging adsorbed CO2 with flowing Ar removed part of the adsorbed CO2, weakly adsorbed species, as spectrum (iii) shown by partial return of the IR features of pretreated sorbents. The remaining adsorbed CO2 was completely desorbed after TPD in spectrum (iv), as shown by the nearly indistinguishable IR spectra when comparing those spectra obtained after TPD and pretreatment. Subtracting APret. from ACO2/H2O,10m gave the IR spectrum (ii)−(i) of adsorbed CO2 and H2O in Figure 3(c), which exhibited the negative IR bands in N−H stretching at 3367 and 3288 cm−1, C−H stretching in 2925 cm−1 − 2815 cm−1, and positive bands in ammonium ions (i.e., RNH3+/R1R2NH2+), carbamate (NCOO−), NCOOH as well as adsorbed H2O. The formation of ammonium ions, RNH3+/R1R2NH2+, was accompanied by a decrease in C−H intensity. R, R1 and R2 are alkyl groups. It has been suggested that a functional group carried positive charge in RNH3+/ R1R2NH2+ could lead to a decrease in stretching intensity of C−H which is associated with R1/R2. This decrease has been related to changes in the C−H dipole moment.24 Note that the assignment of ammonium ion bands which has been ambiguously reported,25,26 will be further clarified and discussed in Figure 4. Changes in the IR intensity of gaseous CO2 (2360 cm−1), adsorbed CO2 (1326 cm−1) and adsorbed H2O (3470 cm−1) were plotted along with the temperature profiles in Figure 3(a). Adsorption of CO2/H2O led to a rise in the temperature of the sorbent bed, a manifestation of releasing the heat of adsorption. Temperature oscillation around 40 and 120 °C is resulted from time-proportional control. Gaseous CO2, adsorbed CO2, and H2O profiles reflect the adsorption and desorption kinetic behaviors which will be further discussed.

Table 2. IR Band Assignments for Adsorbed CO2 and CO2/H2O Species wavenumber (cm−1)

assignment

6554, 6489, 4925 5255, 5152 3470 3010−2990

N−H combination OH combination OH stretching NH3+ stretching

2750−2410

N−H stretching

2051

NH3+ stretching

1691−1681 1650−1635

COOH stretching NH3+ deformation

1560 1525

COO− stretching NH3+ /NH2+ deformation COO− stretching NH2+ deformation

1488 1411 1385 1358 1326

C−N stretching/ NCOO− skeletal vibration C−O stretching NCOO− skeletal vibration

species amine in TEPA adsorbed H2O adsorbed H2O primary ammonium ions primary/secondary ammonium ions primary ammonium ions carbamic acid primary ammonium ions carbamate primary/secondary ammonium ions carbamate secondary ammonium ions carbamate bicarbonate carbamate

ref 27 28 24, 28 23, 29, 30 23, 29 this work 16, 25, 30, 31 16, 23, 29, 30, 32 16, 29, 30 this work 23, 29 this work 16 16 29, 30, 33

Figure 4. IR intensity profiles (i.e., the IR intensity of a specific band vs time) and absorbance spectra of TS sorbent during flowing (a) HCl/H2O vapor, (b) H2O vapor at 25 °C. Absorbance = log(I0/I), where I0 is the single-beam spectrum of TS sorbent after pretreatment, and I is the singlebeam spectrum taken after exposure to HCl/H2O vapor for 0.04/0.5, 1.0, and 10.0 min. D

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Energy & Fuels HCl/H2O Adsorption/Desorption Study. IR band assignments of ammonium ions (RNH3+/R1R2NH2+), carbamate (NCOO−), and carbamic acid (NCOOH) have been highly speculated due to the overlapping of ammonium ion and carbamate bands. We first elucidated these overlapping bands by identifying the bands associated with ammonium ions through protonating amine with HCl vapor. The HCl/H2O vapor was introduced into inlet line by flowing Ar via a 37 wt % HCl/H2O saturator, which was kept at room temperature. Before HCl probing, the HCl/H2O saturator was bubble with Ar for 10 min to remove gaseous and dissolved CO2. Exposure of the sorbent to HCl/H2O vapor for 0.04 min produced IR bands at 2994, 2725, 2051, and 1635 cm−1, which can be assigned as primary ammonium ions because the formation of these bands was associated with the decrease in the primary amine band at 3367 cm−1,27 shown in Figure 4a. Further exposure of HCl/H2O vapor to 1 min led to the growth of the ammonium ion bands at 4427 and 3010 cm−1 shifted from 2994, 2416, 2051, 1525, and 1411 cm−1. Prolonged exposure of HCl/H2O resulted in an appreciable decrease in the secondary amine band at 3288 cm−1,29 along with the growth of the bands at 4427, 2416, and 1411 cm−1. This is in contrast to the absence of variation in the intensity of primary amine. Thus, these two bands (2416 cm−1 and 1411 cm−1) can be assigned to the secondary ammonium ions. It is interesting to see the sequence of HCl adsorption on amines followed that of CO2, which adsorbed on primary amine first and then on secondary amine. The broad band in the 3200−2500 cm−1 region is attributed to hydrogen bonding between ammonium ions. It is interesting to note that the bands related to adsorbed H2O were not observed, indicating H2O vapor was not able to compete with HCl for the amine sites. Following the band assignment of ammonium ions, it became evident that the 1585, 1560, and 1326 cm−1 bands in Figure 3c resulted from carbamate (NCOO−) and 1681 cm−1 from carbamic acid (NCOOH). Adsorbed H2O on TS sorbent in Figure 4b produced a broad IR band centered at 3470 cm−1, a band with plateau in 3000−3700 cm−1, a bending band at 1650 cm−1, and a skew (asymmetric shape) near-IR band at 5255 cm−1. This skew 5255 cm−1 band consists of two components: the first one centered at 5255 cm−1 could be attributed to the H2O associated with secondary amine, and the second component centered at 5152 cm−1 could be related to the physical adsorbed H2O layers. Increasing H2O exposure led to an increase in the intensity of the 5152 cm−1 shoulder band. This shoulder band has been attributed to hydrogen-bonded water in the form of thin film on the TiO2 surface.28 An increase in this shoulder intensity indicated an accumulation of water on the surface of amine molecules. These accumulated H2O molecules gave IR spectra resembling those of liquid water which gave a broad band in 3200−3600 cm−1. The structure of these adsorbed water molecules is proposed and illustrated in Scheme 3a. The intermixing of H2O and amine caused the formation of ammonium ions and a significant decrease in the N−H stretching intensity. The IR spectra of adsorbed D2O on TEPA in Figure S3 shows the negative secondary amine band at 3288 cm−1 when D2O adsorbed on TEPA film, which did not show a broad OH stretching from H2O in the region of 3000−3500 cm−1, providing unambiguous band assignment for the association of D2O with secondary amine. Figure 4b showed that adsorbed H2O also interacts with the surface isolated − OH groups via hydrogen bonds, which was evidenced by the decrease in the IR intensity of Si−OH at 3710 cm−1.

Scheme 4. Displacement of the Immobilized Amine from Si−OH Surface by Adsorbed H2O

This observation can be illustrated by Scheme 4, which shows adsorbed H2O displaced amines from the surface Si−OH sites, releasing amine functional groups for CO2 capture. Our previous infrared study showed that impregnated amine could displace the hydrogen bonded water from SiO2 surface,30 suggesting the reversibility of water displacement. Water adsorption on the amine sorbent resulted in a more pronounced decrease in the N−H intensity of secondary amine at 3288 cm−1 than of the primary amine at 3367 cm−1. The decrease in the intensity of the secondary amine band was also correlated with that of the negative −CH2− band at 2815 cm−1, as shown in Figure 4b and in the inset. This observation indicated the secondary amine is the preferred site for H2O adsorption (Scheme 3). An amine grafted SiO2 sorbent, CQA12, was tested, and the results, as shown in Figure S4, further confirmed the affinity of secondary amine to H2O. The plateau band below 3288 cm−1 can be attributed to hydrogen bonding resulted from the interaction between O of the H2O molecules and H of the N−H in the TEPA molecules. This band has been commonly observed in the IR spectra of amino acid.32 IR results of Ar flowing and TPD in Figure S5 showed that more than 90% of adsorbed H2O was attributed to weakly adsorbed species. In contrast to adsorbed H2O, adsorbed HCl formed ammonium ions which cannot be removed by TPD, revealing that HCl was irreversibly adsorbed on amine, poisoning the amine sites. CO2/H2O Adsorption/Desorption Study. Figure 5a/b compares the kinetic behaviors of adsorbed species during CO2 and CO2/H2O adsorption on TS sorbent. The IR spectra of CO2 and CO2/H2O adsorption on the amine sorbent in the first 0.1 min were nearly identical, giving the ammonium carbamate bands. The absence of adsorbed H2O during this period indicated that CO2 prior to H2O adsorbed on sorbent during exposure to CO2/H2O. Prolonging exposure of CO2/ H2O produced an appreciable decrease in the negative band of secondary amine at 3288 cm−1. The change can be highlighted by the difference in IR spectra obtained by subtracting the spectrum at 1 min from 10 min, as shown in the inset of Figure 5. The effect of adsorbed H2O can be further manifested by comparing the IR spectra of adsorbed CO2 produced between 1 and 10 min during which time a significant amount of H2O was accumulated on the surface of the sorbent. Accumulated H2O gave a pronounced near-IR shoulder band at 5152 cm−1 and the plateau band from 3710 to 3288 cm−1. We have attributed the 5152 cm−1 band to hydrogen-bonded water in the form of thin film on the surface of amine sorbent (Figure 4 and Scheme 3a). An appreciable decrease in the secondary amine at 3288 cm−1 suggested that adsorbed H2O, which mainly associated with the secondary amine, caused an increase in CO2 capture capacity. The enhancement of CO2 capture capacity can be related to the following two observations. First, water displaced the amine E

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Figure 6. IR difference spectra of TS sorbent obtained by subtracting the spectrum of Ar flow at 0.3 min from 10 min of CO2 and CO2/H2O adsorption, given by ACO2,10m − AAr,0.3m and ACO2/H2O,10m − AAr,0.3m. Absorbance = log(I0/I), where I0 is the single-beam spectrum of the TS sorbent after pretreatment; I is the single-beam spectrum taken after flowing CO2 and CO2/H2O for 10 min, flowing Ar for 0.3 min.

Figure 5. IR intensity profiles (i.e., the IR intensity of a specific band vs time) and absorbance spectra of adsorbed species on TS sorbent during flowing (a) CO2/H2O and (b) CO2. Absorbance = log(I0/I), where I0 is the single-beam spectrum of the TS sorbent after pretreatment; I is the selected single-beam spectrum taken during flowing CO2 and CO2/H2O. The difference spectra in the inset were obtained by subtracting the IR spectra at 1.0 min from 10 min.

sites which were associated with the −OH of SiO2 surface via hydrogen bonding (Scheme 4). This displacement was evidenced by the decrease in the IR intensity of isolated −OH at 3710 cm−1 in Figure 4b. Furthermore, the reported binding strength of hydrogen bonds for N−H to O−H (8 kJ/mol) is smaller than that of O−H to O−H (21 kJ/mol).34 Thus, the released amines could be available for CO2 capture in the presence of H2O vapor. Another factor contributing to the enhancement of CO2 capture capacity was the increased amount of carbamic acid in CO2/H2O adsorption. Carbamic acid, which gave a shoulder band at 1691 cm−1 in the absorbance spectra during CO2 and CO2/H2O adsorption, was clearly visible in the difference spectrum in Figure 6. Promoting the formation of carbamic acid is a cost-effective approach for enhancing CO2 capture of amine sorbents due to the 1:1 stoichiometry of CO2: amine. The IR intensity profiles in the left panel of the Figure 5a show that the formation of the adsorbed CO2 species (i.e., ammonium carbamate pairs) prior to that of the adsorbed H2O. Adsorbed H2O further slowed down the formation of adsorbed CO2 species. The formation rate of adsorbed species followed the order of carbamate > ammonium ions > carbamic acid > adsorbed H2O. Carbamate formation prior to that of the ammonium ions suggested that the reaction of CO2 with amine proceeded through the zwitterion intermediate. As illustrated in Schemes 1 and 2, the formation of ammonium ion−carbamate pair requires a neighboring amine to accept the proton deprotonated from zwitterion (NH2+COO−) intermediates, which were produced upon CO2 attachment on amine sites.35 Figure 7 shows the IR spectra of Tads, Sads, and Wads. Tads (i.e., total adsorbed species) were produced from exposure of the

Figure 7. IR intensity profiles (i.e., the IR intensity of a specific band vs time) and absorbance spectra of total adsorbed (Tads), strongly adsorbed (Sads), and weakly adsorbed (Wads) (a) CO2/H2O and (b) CO2. IR spectrum for Tads was obtained by ACO2/H2O,10m − APret and ACO2,10m − APret. Sads = ATPD,120°C − AAr,10m and Wads = AAr,0.5m − AAr,10m. Absorbance = log(I0/I), where I0 is the single-beam spectrum of the TS sorbent after pretreatment; I is the single-beam spectrum taken after flowing CO2 and CO2/H2O for 10 min, flowing Ar for 0.5 and 10 min, and TPD at 120 °C.

sorbent to CO2 and CO2/H2O flow for 10 min, respectively. These spectra were obtained by subtracting the spectrum of pretreatment (APret) from 10 min of CO2 and CO2/H2O adsorption (ACO2,10m and ACO2/H2O,10m), given by Tads.CO2 = ACO2,10m − APret and Tads.CO2/H2O = ACO2/H2O,10m − APret. Adsorbed H2O resulted in a subtle change in the spectra of strongly adsorbed CO2 (Sads.CO2) and CO2/H2O (Sads.CO2/H2O) as well as weakly adsorbed CO2 (Wads.CO2) and CO2/H2O (Wads.CO2/H2O). Zooming into the spectra in the 2000−3800 cm−1 F

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strength of carbamate formed on primary amine sites.37 These studies further supporting our proposition on the formation of paired carbamic acid in CO2 and CO2/H2O adsorption. A comparison of the derivative intensity profiles of adsorbed species presented on the right side of Figure 8 shows that flowing CO2/H2O produced adsorbed species which require higher temperature to be desorbed. Table 3 lists the binding energy of strongly adsorbed species which were estimated by first-order (1st order) and

region, shown in the inset of Figure 7, revealed that weakly adsorbed species (Wads) give a more intense negative secondary amine band, while the strongly adsorbed species (Sads) gave a more intense negative primary amine band. These results suggested that weakly adsorbed species are associated with secondary amine and strongly adsorbed species are associated with primary amine. The left panels in Figure 7 compare the IR intensity profiles of the adsorbed species during desorption. Desorption of adsorbed species followed the sequence of carbamic acid > adsorbed H2O > ammonium carbamate. Purging the adsorbed species with flowing Ar removed more than 80% of carbamic acid and more than 90% of adsorbed H2O. The weakly adsorbed CO2 requiring low thermal energy to be removed will play a major role in pressure swing adsorption/desorption (PSA). A significant fraction of weakly adsorbed CO2 is attributed to carbamic acid which can be observed by the difference spectra, ACO2,10m − AAr,0.3m and ACO2/H2O,10m − AAr,0.3m, shown in Figure 6. These spectra represent those species which were removed from the sorbent during the first 0.3 min of Ar flowing. The positive carbamic acid band at 1691 cm−1 and the negative secondary amine band at 3288 cm−1 further indicated that carbamic acid is primarily associated with secondary amine (Scheme 3b). The presence of H2O further enhanced the carbamic acid intensity, suggesting that the formation of carbamic acid could be facilitated by the transfer of a proton from ammonium ion to carbamate via an adjacent H2O (Scheme 3b). Figure 8 compares thermal desorption behaviors of strongly adsorbed CO2 and CO2/H2O during temperature-programmed

Table 3. Calculation of Binding Energy of Adsorbed Species Edes (kJ/mol) adsorbed species

1st order

2nd order

paired carbamic acid (1681 cm−1) carbamate (1326 cm−1) ammonium ions (1525 cm−1) paired carbamic acid (1681 cm−1) carbamate (1326 cm−1) ammonium ions (1525 cm−1) adsorbed H2O (3470 cm−1)

20.0 21.3 21.1 19.9 20.5 20.3 19.6

84.7 93.2 92.8 86.2 90.2 88.7 85.4

flue gas 15% CO2

15% CO2 + H2O

second-order (2nd order) TPD equations.38 This is a simplified method to obtain the binding energy of adsorbed species by the easily accessible spectral features, the maximum desorption temperature (Tmax) and the peak width at half-maximum intensity (W1/2), as the first- and second-order equations shown. It is interesting to note that the second-order equation produced the valid values which suggest that the desorption of strongly adsorbed species could follow second-order kinetics. Furthermore, the derivative intensity profiles also follow those produced by typical second-order desorption kinetics. The results of this study are applicable to the interpretation of CO2 capture behavior of many reported amine sorbents containing both primary and secondary amine. For the pressure swing adsorption process, it would be desirable to develop the secondary amine-enriched sorbents. For the temperature swing adsorption process, the presence of H2O did not significant vary the binding energy of the strongly adsorbed species. 1st order equation: Edes = −1 + RTmax

⎛ W1/2 ⎞2 5.832Tmax ⎜ ⎟ + W1/2 ⎝ Tmax ⎠

2nd order equation: ⎡ ⎛ Edes 3.117Tmax 2 ⎞⎤ ⎟⎟⎥ = 2⎢ −1 + ⎜⎜1 + ⎢⎣ RTmax W1/2 2 ⎠⎥⎦ ⎝

Figure 8. IR intensity profiles (i.e., the IR intensity of a specific band vs time) and the derivative intensity (ΔI/Δt) profiles of adsorbed species on TS sorbent produced from desorption of (a) adsorbed CO2/H2O and (b) adsorbed CO2 species during TPD with Ar flowing.

where Edes is the binding energy of adsorbed species; R is the gas constant; Tmax is the peak maximum desorption temperature; W1/2 is the peak width at half of the maximum intensity.

desorption (TPD). Note that less than 20% of carbamic acid was unable to desorb during 10 min of Ar flowing. The carbamic acid species at 1681 cm−1 has a higher binding energy than adsorbed H2O. These carbamic acid species could be in a paired form associated by hydrogen bonding, as shown in Scheme 3b. The hydrogen bonding between CO and OH could shift the wavenumber of CO from 1691 cm−1 in a single carbamic acid to 1681 cm−1 in paired carbamic acid. A recent DFT calculation showed that the binding energy of paired carbamic acid on primary amine sites is more than 130 kJ/mol,36 which is comparable to a 150 kg/mol binding

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CONCLUSIONS This study clarifies the structure and IR band assignments of adsorbed CO2 and H2O on amine sites, reveals the roles of H2O playing on the amine sorbent surface, and elucidates the nature of various forms of adsorbed CO2/H2O. CO2 adsorbs on primary amine as ammonium carbamate and on secondary amine as carbamic acid. H2O adsorbed on secondary amine promotes the formation of carbamic acid, resulting in the G

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Energy & Fuels

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increase of the CO2 capture capacity and amine efficiency. The binding energy of the adsorbed species increases in the following order: carbamic acid < adsorbed H2O < paired carbamic acid; ammonium carbamate. A large fraction of these strongly adsorbed species can only be desorbed by temperature-programmed desorption. The observed kinetic behaviors of strongly and weakly adsorbed CO2, along with the knowledge of the sites for binding these species provide a scientific basis for the design of sorbents targeting at either pressure or temperature swing CO2 capture processes.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b01423. The measurement of CO2 capture capacity and amine efficiency are provided in Figure S1, S2 and Table S1. A literature summary of H2O effects on CO2 capture on solid amine sorbent is provided in Table S2. IR spectra of D2O adsorption on TEPA liquid film are in Figure S3. IR spectra of CO2 and CO2/H2O adsorption on aminegrafted sorbent are presented in Figure S4, and IR intensity profiles and absorbance spectra of TS sorbent during Ar purging and TPD after adsorption of HCl and H2O are in Figure S5 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +1 330-972-6993. Fax: +1 330-972-5290. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Mr. Hailiang Jin for his help in drawing the schemes. This work is supported by the U.S. Department of Energy under grants DE-FE0013127 and Ohio Coal Development Office under grant R-13-16.

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REFERENCES

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DOI: 10.1021/acs.energyfuels.6b01423 Energy Fuels XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.energyfuels.6b01423 Energy Fuels XXXX, XXX, XXX−XXX